Better Boilers adds easy-to-use, multiblock steam boilers back into Minecraft 1.12. Shape boilers however you want, add vents and valves, put a firebox on bottom, and add a controller, and you're good to go.
Boilers can contain up to 1,000 blocks (including boiler components and firebox components). Each boiler component (boiler blocks, boiler valves, or boiler vents) adds one bucket of capacity to the water tank and half a bucket of capacity to the steam tank. Each firebox component (firebox blocks or firebox hatches) used speeds up steam production, but also increases fuel cost.
This mod is not viable to use on its own. There are no fluid pipes, no RF generation, and no metals for making the boiler blocks. Better Boilers is designed to be used alongside mods like Thermal Expansion or Immersive Engineering addon Immersive Tech, which both add steam-based power production. Boiler blocks are made using brass plates, but will fall back to using brass ingots, constantan plates (added by both Thermal Expansion and Immersive Engineering), or iron and gold ingots depending on what ore dictionary entries are available. If you would like to use the standard recipe, use a mod like Smores or Base Metals to add Brass plates externally.